The invention relates to a fuel injection method for an internal combustion engine, especially of a motor vehicle.
In internal combustion engines, a high compression ratio is sought for reducing the consumption and for increasing the torque. However, with increasing compression ratio, the problem of an uncontrolled occurring self-ignition of the air/fuel mixture increases. A knocking combustion occurs as a consequence. One distinguishes between two types of knocking, the so-called acceleration knocking at low engine speed and high load (can be heard as a ringing) as well as the high engine speed knocking (which cannot be heard) at high engine speeds and high load. The high engine speed knocking is especially critical for the engine.
The knocking combustion generates pressure oscillations which superpose on the normal pressure course.
Continuous knocking causes severe damage to the engine (destroyed cylinder head seals, bearing damage, holes in the piston) and damage to the spark plugs.
The knocking limit is dependent, inter alia, on the type of engine and on the fuel. The knocking resistance of fuels for fuel-injection engines is characterized by the octane number (ON). The higher this number is, the more resistant to knocking is the fuel.
The oscillations emanating from the combustion chamber are detected by the knock sensor. The output signal of the knock sensor is supplied to a control apparatus of the engine. The control apparatus evaluates the knock signal and initiates suitable countermeasures when a knocking combustion is detected. For this purpose, the ignition time point is, as a rule, shifted so far in the direction xe2x80x9cretardxe2x80x9d until there is again a drop below the knock limit.
The present invention has as its object to improve a method of the above type so that an operation of an internal combustion engine is possible at its operating limits without suffering a loss in torque because of knocking combustion.
The method of the invention is for injecting fuel for an internal combustion engine including an engine of a motor vehicle. The method includes the steps of: making a first injection in an induction stroke; determining a tendency-to-knock signal characterizing the danger of an occurring knocking; and, making at least one second injection when the tendency-to-knock signal exceeds a threshold value.
The especially great advantage of the present invention is that an increase of the maximum outputted torque of the engine is reached. Furthermore, the knocking limit is raised, that is, the engine becomes more resistant to knocking.
A further advantage is that, with a knock control with the aid of double injections, only a slight torque reduction results compared to the knock control with the aid of a shift of the ignition angle in the direction of retard. A knock control via double injection and ignition angle intervention or a combined intervention of both is possible in order to operate the engine at the optimal operating point.